1. Field of the Invention
This invention relates to an electron emission device, particularly to one comprising a P-type semiconductor layer formed on a N-type semiconductor layer which emitts electrons injected into said P-type semiconductor layer by utilizing the negative electron affinity state.
2. Related Background Art
Among the electron emission devices of the prior art, there is the system in which a work function lowering material layer is formed on a P-type semiconductor layer and electrons are emitted by utilizing the NEA (negative electron affinity) at which the vacuum level is at an energy level lower than the conduction band of the P-type semiconductor.
FIG. 1(A) is a schematic illustration of the electron emission device by use of the NEA state, and FIG. 1(B) a graph showing its schematic current-voltage characteristic.
In the same Figure (A), when a forward bias voltage is applied to the PN junction, the current I flows in the forward direction as shown in the same Figure (B), and a part of the electrons injected from the N layer 8 into the P layer 9 are emitted from the surface of the P layer 9 into vacuum.
On the surface of the P layer 9, a work function lowering material 10 such as of an alkali metal (e.g. Cs), etc. is formed for imparting the NEA state as described above, and the electrons injected into the P layer 9 can be readily emitted, to provide an electron emission device having high electron emission efficiency.
However, in the electron emission device of the prior art as described above, the electron emission efficiency was not sufficient, and it has been desired to have an electron emission device having higher efficiency.